Image capturing device and image capturing method

ABSTRACT

The present invention is applied for an image capturing device having a light source and a camera that captures an image of a measurement subject placed in an optical path that lies between said camera itself and said light source. The image capturing device according to the present invention includes a control unit that subtracts a plurality of frame images captured by said camera during an OFF period of said light source from a plurality of frame images captured by said camera during an ON period of said light source, the number of frame images captured by said camera during the OFF period being the same as that number of frame images obtained by said camera during the ON period and integrates the differences between their images.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2011-070058, filed on Mar. 28, 2011, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image capturing device that isprovided with a light source and a camera and in which a measurementsubject is placed in an optical path that lies therebetween, inparticular, to an image capturing device that captures an image of ameasurement subject in synchronization with ON/OFF states of the lightsource.

2. Description of the Related Art

Image capturing devices that are provided with a light source and acamera and in which a measurement subject is placed in an optical paththat lies therebetween are known. In these image capturing devices, whenan image of a measurement subject is captured in synchronization withthe ON/OFF states of the light source (so-called lock-in imagecapturing), not only images caused by radiations other than the lightsource can be eliminated, but also low-frequency noises such as 1/fnoises can be set to the off setting.

For example, as presented in Non-patent Literature 1 (A. W. M. Lee etal., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL 18, NO. 13, Jul. 1, 2006, p.1415-1417), when a THz wave emitted by a THz light source that isperiodically turned ON and OFF is transmitted to a camera whosesensitivity is within a range from the infrared region to the THz(terahertz) region, the camera detects an electromagnetic wave in whichinfrared and a periodic THz wave have been mixed and captures an imagebased on the detected electromagnetic wave. By calculating thedifference between image data captured by the camera during the ONperiod of the THz light source and image data captured by the cameraduring the OFF period thereof, infrared images can be set off andthereby only THz images can be obtained. As a result, a filter thatsuppresses infrared waves and passes through only THz waves can beomitted.

A specific arrangement of the foregoing image capturing device is shownin FIG. 1.

As shown in FIG. 1, light of a THz wave at a frequency of 4.3 THz (witha wavelength of 70 μm) is emitted by QCL (Quantum Cascade Laser) 102that is mounted on cooler 101. The light travels to off-axis parabolicmirror 103. Off-axis parabolic minor 103 collimates the light andreflects the collimated light to off-axis parabolic mirror 104. Off-axisparabolic mirror 104 radiates the collimated light to envelop(measurement subject) 105. The light that has passed through envelop 105is collected by Si lens 106 to microbolometer array sensor 108 that has320×240 pixels and that is mounted on microbolometer camera 107 (at aframe rate of 60 Hz). Microbolometer array sensor 108 captures an imagebased on the collected light of the THz wave.

FIG. 2 shows a lock-in image capturing method for the image capturingdevice shown in FIG. 1.

As shown in FIG. 2, QCL 102 is turned ON at frame 1 and QCL 102 isturned OFF at frame 2 and frame 3.

At frame 1, an infrared wave and THz wave emitted by QCL 102 arecollected to microbolometer array sensor 108 and thereby an image iscaptured based on both the infrared and THz wave, whereas at frame 2 andframe 3, only infrared waves are collected to microbolometer arraysensor 108 and thereby an image is captured based on only the infrared.

Thus, by calculating the difference between the image data of frame 1and the image data of frame 3, a THz image can be obtained as an imagecaptured based on only the THz wave emitted by QCL 102.

Frame 2 is not used in order to increase the difference between imagedata of frame 1 and image data of frame 3 since a sufficiently long timeelapses after the image data of frame 1 has been captured compared to athermal time constant of around 13 msec of microbolometer array sensor108.

FIG. 3 shows THz image 110 that is captured from envelop 105 thatcontains paper 109 with letters “MIT” written with a pencil at a framerate of 20 Hz based on the method shown in FIG. 2. Thus, letters “MIT”can be checked out without necessity of opening envelop 105.

However, in the foregoing image capturing device, since only thedifference between one piece of image data captured during the ON periodof the light source and one piece of image data captured during the OFFperiod thereof, as a problem that would arise, the signal-to-noise ratiocould not be improved as expected.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide an imagecapturing device and an image capturing method that can solve theforegoing problem.

A first image capturing device according to the present invention is animage capturing device having a light source and a camera that capturesan image of a measurement subject placed in an optical path that liesbetween said camera itself and said light source, comprising:

a control unit that subtracts a plurality of frame images captured bysaid camera during an OFF period of said light source from a pluralityof frame images captured by said camera during an ON period of saidlight source, the number of frame images captured by said camera duringthe OFF period being the same as that number of frame images obtained bysaid camera during the ON period and integrates the differences betweentheir images.

A second image capturing device according to the present invention is animage capturing device having a light source and a camera that capturesan image of a measurement subject placed in an optical path that liesbetween said camera itself and said light source, comprising:

a control unit that integrates a plurality of frame images captured bysaid camera during an ON period of said light source, integrates aplurality of frame images captured by said camera during an OFF periodof said light source, the number of frame images captured by said cameraduring the ON period being the same as the number of frame imagescaptured by said camera during the OFF period, subtracts an integratedimage of the frame images captured by said camera during the OFF periodof said light source from an integrated image of the frame imagescaptured by said camera during the ON period of said light source, andobtains the difference between their images.

A first image capturing method according to the present invention is animage capturing method for an image capturing device having a lightsource and a camera that captures an image of a measurement subjectplaced in an optical path that lies between said camera itself and saidlight source, comprising:

subtracting a plurality of frame images captured by said camera duringan OFF period of said light source from a plurality of frame imagescaptured by said camera during an ON period of said light source, thenumber of frame images captured by said camera during the OFF periodbeing the same as the number of frame images captured by said cameraduring the ON period, and integrating the differences between theirimages.

A second image capturing method according to the present invention is animage capturing method for an image capturing device having a lightsource and a camera that captures an image of a measurement subjectplaced in an optical path that lies between said camera itself and saidlight source, comprising:

integrating a plurality of frame images captured by said camera duringan ON period of said light source;

integrating a plurality of frame images captured by said camera duringan OFF period of said light source, the number of frame images capturedby said camera during the OFF period being the same as the number offrame images captured by said camera during the ON period; and

subtracting an integrated image of the frame images captured by saidcamera during the OFF period of said light source from an integratedimage of the frame images captured by said camera during the ON periodof said light source and obtaining the difference between their images.

According to the first image capturing device and the first imagecapturing method of the present invention, a plurality of frame imagescaptured during an OFF period of a light source are subtracted from aplurality of frame images captured during an ON period thereof, thenumber of frame images captured during the ON period being the same asthe number of frame images captured during the OFF period, and then thedifferences between their images are integrated.

According to a second image capturing device and a second imagecapturing method of the present invention, an integrated image of aplurality of frame images captured during an OFF period of a lightsource is subtracted from an integrated image of a plurality of frameimages captured during an ON period thereof, the number of frame imagescaptured during the OFF period being the same as the number of frameimages captured during the ON period, and the difference between theimages is obtained.

Thus, as an effect of the present invention, the signal-to-noise ratiocan be improved compared to the related art in which the differencebetween one image captured during an ON period of a light source and oneimage captured during an OFF period thereof is calculated and thedifference between the images is obtained.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a structure of an image capturingdevice according to a related art;

FIG. 2 is a schematic diagram describing a method to obtain thedifference between their images for the image capturing device accordingto the related art;

FIG. 3 is a schematic diagram showing a THz transmission image capturedby the image capturing device according to the related art;

FIG. 4 is a schematic diagram showing a structure of an image capturingdevice according to an exemplary embodiment of the present invention;

FIG. 5 is a schematic diagram describing a pulse sequence used in theimage capturing device according to the exemplary embodiment of thepresent invention;

FIG. 6A is a schematic diagram describing a function that compensatesphases of image data for the image capturing device according to theexemplary embodiment of the present invention;

FIG. 6B is a schematic diagram describing the relationship among animage capturing timing signal, an external IF output timing signal, andan image obtaining timing signal for the image capturing deviceaccording to the exemplary embodiments of the present invention;

FIG. 7A is a schematic diagram describing a first method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource, images captured during an OFF period thereof, and thedifferences between their images are stored;

FIG. 7B is a schematic diagram describing a second method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource, images captured during an OFF period thereof, and thedifferences between their images are stored;

FIG. 7C is a schematic diagram describing a third method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource, images captured during an OFF period thereof, and thedifferences between their images are stored;

FIG. 7D is a schematic diagram describing a fourth method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource, images captured during an OFF period thereof, and thedifferences between their images are stored;

FIG. 7E is a schematic diagram describing a fifth method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource, images captured during an OFF period thereof, and thedifferences between their images are stored;

FIG. 7F is a schematic diagram describing a method for an imagecapturing device according to an exemplary embodiment of the presentinvention in which images captured during an ON period of a THz lightsource and images captured during an OFF period thereof are stored;

FIG. 8 is a schematic diagram describing a sample used for an imagecapturing device according to an exemplary embodiment of the presentinvention;

FIG. 9 is a schematic diagram showing a difference in an image in whichan integrated image of a plurality of images of the sample shown in FIG.8 captured during an OFF period of a THz light source is subtracted froman integrated image of a plurality of images of the sample shown in FIG.8 captured during an ON period thereof for an image capturing deviceaccording to an exemplary embodiment of the present invention; and

FIG. 10 is a schematic diagram showing a difference between images inwhich one image captured from the sample shown in FIG. 8 during an OFFperiod of a THz light source is subtracted from one image captured fromthe sample shown in FIG. 8 during an ON period thereof for the imagecapturing device according to the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, with reference to the accompanying drawings, exemplary embodimentsof the present invention will be described.

FIG. 4 shows a structure of an image capturing device according to anexemplary embodiment of the present invention.

As shown in FIG. 4, the image capturing device according to thisexemplary embodiment has THz light source 1, THz camera 4, 1/n frequencymultiplying circuit 6, high voltage pulse power supply 8, AND circuit10, drive circuit 12, and image data obtaining device 13. Image dataobtaining device 13 has CPU 14, buffer 15, and phase compensationcircuit 16.

In FIG. 4, CPU 14 is an example of a control unit; high voltage pulsepower supply 8 is an example of a first pulse circuit; and 1/n frequencymultiplying circuit 6 is an example of a second pulse circuit.

THz wave 2 emitted by THz light source 1 is radiated to sample(measurement subject) 3 and then detected as a reflected wave ortransmitted wave and captured as an image by THz camera 4. According tothis exemplary embodiment, it is assumed that THz wave 2 is detected asa reflected wave.

Sync signal (image capturing timing signal) 5 that represents an imagecapturing timing of THz camera 4 is input from THz camera 4 to 1/nfrequency multiplying circuit 6. 1/n frequency multiplying circuit 6multiplies the frequency of sync signal 5 by 1/n and thereby generatespulse sequence 7 (second pulse sequence).

Pulse sequence 7 is input to AND circuit 10 along with pulse sequence 9(first pulse sequence) supplied from high voltage pulse power supply 8.AND circuit 10 generates new pulse sequence 11 (third pulse sequence)based on pulse sequence 7 and pulse sequence 9.

Pulse sequence 11 is input to drive circuit 12 and then drive circuit 12causes THz light source 1 to oscillate (drive) based on pulse sequence11.

THz camera 4 causes the ON/OFF periods of THz light source 1 tosynchronize with image capturing timings of THz camera 4 so as toperform lock-in image capturing for sample 3. Image data of frame imagescaptured by THz camera 4 (hereinafter frame images are simply referredto as images) are recorded to image data obtaining device 13.

Here, a specific working example of the image capturing device accordingto this exemplary embodiment will be described.

THz light source 1 is a QCL (Quantum Cascade Laser) that radiates anemission line at a frequency of 3.1 THz.

Pulse sequence 9 that is supplied from high voltage pulse power supply 8has a pulse width of 300 nsec and a repetition frequency of 1 kHz.

Sync signal 5 that is output from THz camera 4 is a square wave at afrequency of 60 Hz.

1/n frequency multiplying circuit 6 multiplies the frequency of syncsignal 5 by 1/n (where n is any value that can be set) and therebygenerates square pulse sequence 7 at a frequency of, for example, 15 Hz,7.5 Hz, or 3.75 Hz. The frequency of pulse sequence 7 is referred to asthe lock-in frequency.

Pulse sequence 7 is input to AND circuit 10 along with pulse sequence 9.Thereafter, AND circuit 10 performs an AND operation for pulse sequence7 and pulse sequence 9 and thereby generates pulse sequence 11. Drivecircuit 12 causes THz light source (QCL) 1 to oscillate based on pulsesequence 11. FIG. 5 shows an example that the lock-in frequency is 3.75Hz.

As shown in FIG. 5, when the frequency of pulse sequence 7 is 3.75 Hz,eight images are captured by THz camera 4 at a frame rate of 60 Hzduring an ON period of 133.3 msec of THz light source (QCL) 1 and thecaptured image data are recorded to image data obtaining device 13 so asto perform an image process, for example, an integration process. In thefollowing, it is assumed that the lock-in frequency is 3.75 Hz.

Here, with reference to FIG. 6A and FIG. 6B, functions of phasecompensation circuit 16 disposed in image data obtaining device 13 willbe described.

As shown in FIG. 6A, THz camera 4 has THz array sensor 17, read outintegrated circuit (ROIC) 18, compensation circuit 19, buffer 20, andexternal IF (interface) 21 such as a USB (Universal Serial Bus).

In THz camera 4, ROIC 18 that is a member of THz array sensor 17 obtainsimage data captured by THz array sensor 17 in synchronization with syncsignal 5. As a result, image data are updated (hereinafter the imagedata are referred to as internal image data).

Moreover, in THz camera 4, compensation circuit 19 performs acompensation process for image data so as to alleviate influence ofvarious disturbances such as sensitivity fluctuations of individualsensor elements of THz array sensor 17 and those caused by fluctuationsof environmental temperatures.

Compensation circuit 19 outputs compensated image data to the outside ofTHz camera 4 (hereinafter, this image data are referred to as externaloutput image data). Thus, the update timing of the external output imagedata differs from that of the internal image data. Since the externaloutput image data are output to external IF 21 through buffer 20disposed in THz camera 4 so as to prevent data from being lost, theupdate timing of the external output image data further deviates. Forexample, when the compensation process is performed using two buffers ofa data update writing buffer and an external IF output read buffer, theupdate timing deviates for one frame along with the delay caused by thecompensation process of compensation circuit 19.

FIG. 6B shows an example of a delay between sync signal 5 and externalIF output timing signal 22. Here, external IF output timing signal 22shown in FIG. 6B is a signal that represents the update timing ofexternal output image data. Due to this delay, when the external outputimage data that are output from THz camera 4 are supplied to CPU 14 insynchronization with sync signal 5, an image that lies between frames orimage data with a delay of a frame are obtained and thereby image dataobtaining device 13 cannot accurately perform the image process as aproblem that arises.

To solve this problem, according to this exemplary embodiment, phasecompensation circuit 16 compensates the difference between the phase ofsync signal 5 and the phase of the external output image data.Specifically, phase compensation circuit 16 generates image obtainingtiming signal 23 such that sync signal 5 delays for a predetermined time{(compensation process time of compensation circuit 19)+(update time ofbuffer 20)}. CPU 14 obtains external output image data from THz camera 4based on image obtaining timing signal 23.

Next, with reference to FIG. 7A to FIG. 7E, examples of methods in whichCPU 14 disposed in image data obtaining device 13 stores image datacaptured during an ON period of THz light source (QCL) 1, image datacaptured during an OFF period thereof, and image data representing thedifferences between their images to buffer 15, will be described.

In a first method shown in FIG. 7A, three types of buffers X, Y, and Zare prepared. Eight pieces of image data captured during an ON period ofTHz light source 1 are stored in eight buffer memories X₁˜X₈ thatcompose buffer X; eight pieces of image data captured during an OFFperiod thereof are stored in eight buffer memories Y₁˜Y₈ that composebuffer Y; and then image data representing the differences between theirimages are stored in eight buffer memories Z₁˜Z₈ that compose buffer Z.

In a second method shown in FIG. 7B, two types of buffers X and Y areprepared. Eight pieces of image data captured during an ON period of THzlight source 1 are stored in eight buffer memories X₁˜X₈ that composebuffer X; eight pieces of image data captured during an OFF period arestored in eight buffer memories Y₁˜Y₈ that compose buffer Y; and thenimage data representing the differences between their images are storedin the eight buffer memories X₁˜X₈ that compose buffer X again.

In a third method shown in FIG. 7C, two types of buffers are prepared.Eight pieces of image data captured during an ON period of THz lightsource 1 are stored in eight buffer memories X₁˜X₈ that compose bufferX; eight pieces of image data captured during an OFF period are storedin eight buffer memories Y₁˜Y₈ that compose buffer Y; and then imagedata representing the differences between their images are stored ineight buffer memories Y₁˜Y₈ that compose buffer Y again.

In a fourth method shown in FIG. 7D, two types of buffers X and Y areprepared. However, although the number of buffer memories that composeone buffer X is eight, as in the foregoing second and third methods, thenumber of buffer memories that compose the other buffer Y is two, unlikethe foregoing second and third methods. In other words, eight pieces ofimage data captured during an ON period of THz light source 1 are storedin eight buffer memories X_(i) (where i=1, 2, . . . , 8) that composebuffer X. Thereafter, a first piece of image data captured during an OFFperiod is stored in first buffer memory Y₁ that composes buffer Y, thedifference between the first piece of image data captured during the OFFperiod and the first piece of image data captured during the ON periodis calculated and then image data representing the difference betweenimages is stored in first buffer memory X₁ that composes buffer X. Whilethe calculation and storing procedures are being performed, a secondpiece of image data captured during the OFF period is stored in secondbuffer memory Y₂ that composes buffer Y, the difference between thesecond piece of image data captured during the ON period and the secondpiece of image data captured during the OFF period is calculated, andthen image data representing the difference between images are stored insecond buffer memory X₂ that composes buffer X. This calculation andstoring procedures are performed alternately using two buffer memoriesY₁ and Y₂ that compose buffer Y. As a result, calculation results ofX_(2n+1)−Y₁ (where n=0, 1, 2, 3) are stored in X_(2n+1), whereas thecalculation results of X_(2n)−Y₂ (where n=1, 2, 3, 4) are stored inX_(2n).

In a fifth method shown in FIG. 7E, eight pieces of image data capturedduring an ON period of THz light source 1 and eight pieces of image datacaptured during an OFF period thereof are stored in buffer 1 and imagedata representing the differences between their images are stored inbuffer 2.

CPU 14 integrates image data representing the difference between imagesobtained as described above and displays integrated image datarepresenting the difference between images.

Alternatively, CPU 14 may perform a method in which image data areintegrated before differences between images are obtained instead of amethod in which image data are integrated after image data representingdifferences between images are obtained.

In other words, CPU 14 integrates a plurality of pieces of image datacaptured during an ON period of THz light source (QCL) 1, integrates aplurality of pieces of image data captured during an OFF period thereof,calculates the differences between the integrated image data of theplurality of pieces of image data captured during the ON period and theintegrated image data of the plurality of pieces of image data capturedduring the OFF period, and then displays the difference between imagesof the integrated image data.

With reference to FIG. 7F, such an example in which image data capturedduring an ON period of THz light source (QCL) 1 and image data capturedduring an OFF period thereof are stored in buffer 15 disposed in imagedata obtaining device 13 will be described.

However, CPU 14 cannot simultaneously perfoim a method in which imagedata are integrated after image data representing the differencesbetween images are obtained and a method in which image data areintegrated before image data representing the differences between imagesare obtained. Thus, one of these methods needs to have been set for CPU14.

In the method shown in FIG. 7F, two types of buffers X and Y areprepared. Eight pieces of image data captured during an ON period of THzlight source 1 are stored in eight buffer memories X₁˜X₈ that composebuffer X and then eight pieces of image data captured during an OFFperiod thereof are stored in eight buffer memories Y₁˜Y₈ that composebuffer Y.

Next, with reference to FIG. 8, FIG. 9, and FIG. 10, an example in whichintegrated image data representing the differences between images arecalculated in the foregoing manner and thereby the signal-to-noise isimproved two to three times compared to the related art will bedescribed. In the following, it is assumed that image data areintegrated before image data representing the differences between imagesare obtained.

As shown in FIG. 8, sample 3 in which black cloth tape 25 is adhered onan Al tape coated on reflection plate 24 is prepared. An emission lineat a frequency of 3.1 THz that is emitted by THz light source (QCL) 1that is cooled by liquid nitrogen is radiated to partial region 26 ofreflection plate 24. A reflection image whose emission line has beenreflected by reflection plate 24 is captured by THz camera 4 that isprovided with THz array sensor 17 that have 320 * 240 pixels arranged ata pitch of 23.5 μm.

FIG. 9 is a reflection image of an emission line emitted by THz lightsource (QCL) 1 and reflected by reflection plate 24 (the reflectionimage is depicted as an elongated portion nearly at the center of thefigure). The reflection image is formed in an elongated shape because anoff-axis parabolic mirror (not shown) disposed in THz light source (QCL)1 has an aberration. Since the reflectance of black cloth tape 25 islow, low luminance portion 27 appears nearly at the center of theelongated portion. Low luminance portion 27 corresponds to position 28of cloth tape 25.

FIG. 9 shows a difference in an image in which the difference between anintegrated image of seven pieces of the second to the eighth piece ofimage data captured during an ON period of THz light source 1 and anintegrated image of seven pieces of the second to the eighth piece ofimage data captured during an OFF period thereof is calculated at alock-in frequency of 3.75 Hz. The first piece of image data is not usedtaking account of the fact that the time constant of THz array sensor 17is 16 msec and thereby a signal of the first frame might have been notfully raised or lowered during the ON period or OFF period of THz lightsource (QCL) 1. In this case, image data are stored in buffer 15 basedon the method shown in FIG. 7F. In addition, image data of 24 * 24pixels are integrated so as to further improve the signal-to-noiseratio. The signal-to-noise ratio of high luminance portion 29 on theright of a QCL image shown in FIG. 9 is around 400.

FIG. 10 shows a difference in an image between one piece of image datacaptured during an ON period of THz light source (QCL) 1 and one pieceof image data captured during an OFF period thereof so as to check outhow the present invention improves the signal-to-noise ratio compared tothe related art. This difference in the image corresponds to an imageobtained based on a function of the image capturing device according tothe related art. Sample 3 is the same as the sample from which the imageshown in FIG. 9 is captured. The shape of the image shown in FIG. 10 isthe same as that shown in FIG. 9. The lock-in frequency at which thedifferences between image shown in FIG. 10 is obtained is also 3.75 Hz.The signal-to-noise ratio of high luminance portion 29 on the right of aQCL image shown in FIG. 10 is around 135.

Thus, it is clear that the present invention (shown in FIG. 9) canimprove the signal-to-noise ratio two to three times compared to therelated art (shown in FIG. 10).

With reference to the exemplary embodiments, the present invention hasbeen described. However, it should be understood by those skilled in theart that the structure and details of the present invention may bechanged in various manners without departing from the scope of thepresent invention.

According to foregoing exemplary embodiments, successive seven piecesfrom among all eight pieces of image data captured during ON/OFF periodsof THz light source 1 were used. Alternatively, as long as the timeconstant of THz array sensor 17 is low, all eight pieces of image datamay be used. Alternatively, seven pieces from the first to the seventhpiece of image data may be used. In other words, according to thepresent invention, a plurality of successive pieces from among allpieces of image data captured during the ON/OFF periods of THz lightsource 1 may be selected and used.

In addition, according to a foregoing exemplary embodiment, a reflectionimage reflected by sample 3 was obtained. Alternatively, a transmissionimage that transmits through sample 3 may be obtained.

In addition, the foregoing exemplary embodiments deal withelectromagnetic waves of a THz frequency band. Alternatively, thepresent invention can be applied to electromagnetic waves of otherfrequency bands as well as such a frequency band.

The whole or part of the exemplary embodiments disclosed above can bedescribed as but not limited to, the following supplementary notes.

[Supplementary Note 1]

An image capturing device having a light source and a camera thatcaptures an image of a measurement subject placed in an optical paththat lies between said camera itself and said light source, comprising:

a control unit that subtracts a plurality of frame images captured bysaid camera during an OFF period of said light source from a pluralityof frame images captured by said camera during an ON period of saidlight source, the number of frame images captured by said camera duringthe OFF period being the same as that number of frame images obtained bysaid camera during the ON period and integrates the differences betweentheir images.

[Supplementary Note 2]

The image capturing device as set forth in supplementary note 1,

wherein said control unit stores a plurality of frame images captured bysaid camera during the ON period of said light source in a first buffer,

wherein said control unit stores a plurality of frame images captured bysaid camera during the OFF period of said light source in a secondbuffer,

wherein said control unit subtracts the plurality of frame images storedin said second buffer from the plurality of frame images stored in saidfirst buffer and stores the differences between their images in a thirdbuffer, the number of frame images stored in said second buffer beingthe same as the number of frame images stored in said first buffer, and

wherein said control unit integrates the differences between imagesstored in said third buffer.

[Supplementary Note 3]

The image capturing device as set forth in supplementary note 2,

wherein said control unit selects any successive frame images from amongall the frame images captured by said camera during the ON period ofsaid light source, and

wherein said control unit selects successive frame images from among allframe images captured by said camera during the OFF period of said lightsource, the successful number of frame images selected from among allframe images captured during the ON period being the same as thesuccessive number of frame images selected from among all frame imagescaptured during the OFF period.

[Supplementary Note 4]

The image capturing device as set forth in supplementary note 2 or 3,

wherein said control unit uses said first buffer or said second bufferas said third buffer.

[Supplementary Note 5]

The image capturing device as set forth in supplementary note 1,

wherein said control unit stores a plurality of frame images captured bysaid camera during the ON period of said light source and a plurality offrame images captured by said camera during the OFF period of said lightsource in a first buffer, the number of frame images captured by saidcamera during the ON period of said light source being the same as thenumber of frame images captured by said camera during the OFF period ofsaid light source,

wherein said control unit subtracts the plurality of frame imagescaptured by said camera during the OFF period of said light source andstored in said first buffer from the plurality of frame images capturedby said camera during the ON period of said light source and stored insaid first buffer and stores the differences between their images insaid second buffer, the number of frame images captured by said cameraduring the OFF period of said light source and stored in said secondbuffer being the same as the number of frame images captured by saidcamera during the ON period of said light source and stored in saidfirst buffer, and

wherein said control unit integrates the differences between imagesstored in said second buffer.

[Supplementary Note 6]

An image capturing device having a light source and a camera thatcaptures an image of a measurement subject placed in an optical paththat lies between said camera itself and said light source, comprising:

a control unit that integrates a plurality of frame images captured bysaid camera during an ON period of said light source, integrates aplurality of frame images captured by said camera during an OFF periodof said light source, the number of frame images captured by said cameraduring the ON period being the same as the number of frame imagescaptured by said camera during the OFF period, subtracts an integratedimage of the frame images captured by said camera during the OFF periodof said light source from an integrated image of the frame imagescaptured by said camera during the ON period of said light source, andobtains the difference between their images.

[Supplementary Note 7]

The image capturing device as set forth in supplementary note 6,

wherein said control unit selects any successive frame images from amongall the frame images captured by said camera during the ON period ofsaid light source and integrates the selected frame images, and

wherein said control unit selects successive frame images from among allthe frame images captured by said camera during the OFF period of saidlight source, the number of frame images captured by said camera duringthe OFF period being the same as the number of frame images captured bysaid camera during the ON period, and integrates the selected frameimages.

[Supplementary Note 8]

The image capturing device as set forth in any one of supplementarynotes 1 to 7, further comprising:

a first pulse circuit that generates a first pulse sequence;

a second pulse circuit that generates a second pulse sequence based onan image capturing timing signal that represents an image capturingtiming of said camera, said second pulse sequence having a lowerfrequency than does the image capturing timing signal;

an AND circuit that perfonns an AND operation for said first pulsesequence and said second pulse sequence; and

a drive circuit that drives said light source based on a third pulsesequence that is a calculation result of the AND operation of said ANDcircuit.

[Supplementary Note 9]

The image capturing device as set forth in supplementary note 8,

wherein said second pulse circuit is capable of setting any frequencythat is lower than the frequency of said image capturing timing signaland generates said second pulse sequence at the frequency that has beenset.

[Supplementary Note 10]

The image capturing device as set forth in supplementary note 8 or 9,further comprising:

a phase compensation circuit that generates an image obtaining signal ofwhich said image capturing timing signal is delayed for a predeteiminedtime,

wherein said control unit obtains frame images captured by said camerabased on said image obtaining signal.

[Supplementary Note 11]

An image capturing method for an image capturing device having a lightsource and a camera that captures an image of a measurement subjectplaced in an optical path that lies between said camera itself and saidlight source, comprising:

subtracting a plurality of frame images captured by said camera duringan OFF period of said light source from a plurality of frame imagescaptured by said camera during an ON period of said light source, thenumber of frame images captured by said camera during the OFF periodbeing the same as the number of frame images captured by said cameraduring the ON period, and integrating the differences between theirimages.

[Supplementary Note 12]

An image capturing method for an image capturing device having a lightsource and a camera that captures an image of a measurement subjectplaced in an optical path that lies between said camera itself and saidlight source, comprising:

integrating a plurality of frame images captured by said camera duringan ON period of said light source;

integrating a plurality of frame images captured by said camera duringan OFF period of said light source, the number of frame images capturedby said camera during the OFF period being the same as the number offrame images captured by said camera during the ON period; and

subtracting an integrated image of the frame images captured by saidcamera during the OFF period of said light source from an integratedimage of the frame images captured by said camera during the ON periodof said light source and obtaining the differences between their images.

1. An image capturing device having a light source and a camera thatcaptures an image of a measurement subject placed in an optical paththat lies between said camera itself and said light source, comprising:a control unit that subtracts a plurality of frame images captured bysaid camera during an OFF period of said light source from a pluralityof frame images captured by said camera during an ON period of saidlight source, the number of frame images captured by said camera duringthe OFF period being the same as that number of frame images obtained bysaid camera during the ON period and integrates the differences betweentheir images.
 2. The image capturing device as set forth in claim 1,wherein said control unit stores a plurality of frame images captured bysaid camera during the ON period of said light source in a first buffer,wherein said control unit stores a plurality of frame images captured bysaid camera during the OFF period of said light source in a secondbuffer, wherein said control unit subtracts the plurality of frameimages stored in said second buffer from the plurality of frame imagesstored in said first buffer and stores the differences between theirimages in a third buffer, the number of frame images stored in saidsecond buffer being the same as the number of frame images stored insaid first buffer, and wherein said control unit integrates thedifferences between images stored in said third buffer.
 3. The imagecapturing device as set forth in claim 2, wherein said control unitselects any successive frame images from among all the frame imagescaptured by said camera during the ON period of said light source, andwherein said control unit selects successive frame images from among allframe images captured by said camera during the OFF period of said lightsource, the successful number of frame images selected from among allframe images captured during the ON period being the same as thesuccessive number of frame images selected from among all frame imagescaptured during the OFF period.
 4. The image capturing device as setforth in claim 2, wherein said control unit uses said first buffer orsaid second buffer as said third buffer.
 5. The image capturing deviceas set forth in claim 1, wherein said control unit stores a plurality offrame images captured by said camera during the ON period of said lightsource and a plurality of frame images captured by said camera duringthe OFF period of said light source in a first buffer, the number offrame images captured by said camera during the ON period of said lightsource being the same as the number of frame images captured by saidcamera during the OFF period of said light source, wherein said controlunit subtracts the plurality of frame images captured by said cameraduring the OFF period of said light source and stored in said firstbuffer from the plurality of frame images captured by said camera duringthe ON period of said light source and stored in said first buffer andstores the differences between their images in said second buffer, thenumber of frame images captured by said camera during the OFF period ofsaid light source and stored in said second buffer being the same as thenumber of frame images captured by said camera during the ON period ofsaid light source and stored in said first buffer, and wherein saidcontrol unit integrates the differences between images stored in saidsecond buffer.
 6. An image capturing device having a light source and acamera that captures an image of a measurement subject placed in anoptical path that lies between said camera itself and said light source,comprising: a control unit that integrates a plurality of frame imagescaptured by said camera during an ON period of said light source,integrates a plurality of frame images captured by said camera during anOFF period of said light source, the number of frame images captured bysaid camera during the ON period being the same as the number of frameimages captured by said camera during the OFF period, subtracts anintegrated image of the frame images captured by said camera during theOFF period of said light source from an integrated image of the frameimages captured by said camera during the ON period of said lightsource, and obtains the difference between their images.
 7. The imagecapturing device as set forth in claim 6, wherein said control unitselects any successive frame images from among all the frame imagescaptured by said camera during the ON period of said light source andintegrates the selected frame images, and wherein said control unitselects successive frame images from among all the frame images capturedby said camera during the OFF period of said light source, the number offrame images captured by said camera during the OFF period being thesame as the number of frame images captured by said camera during the ONperiod, and integrates the selected frame images.
 8. The image capturingdevice as set forth in claim 1, further comprising: a first pulsecircuit that generates a first pulse sequence; a second pulse circuitthat generates a second pulse sequence based on an image capturingtiming signal that represents an image capturing timing of said camera,said second pulse sequence having a lower frequency than does the imagecapturing timing signal; an AND circuit that performs an AND operationfor said first pulse sequence and said second pulse sequence; and adrive circuit that drives said light source based on a third pulsesequence that is a calculation result of the AND operation of said ANDcircuit.
 9. The image capturing device as set forth in claim 8, whereinsaid second pulse circuit is capable of setting any frequency that islower than the frequency of said image capturing timing signal andgenerates said second pulse sequence at the frequency that has been set.10. The image capturing device as set forth in claim 8, furthercomprising: a phase compensation circuit that generates an imageobtaining signal of which said image capturing timing signal is delayedfor a predetermined time, wherein said control unit obtains frame imagescaptured by said camera based on said image obtaining signal.
 11. Animage capturing method for an image capturing device having a lightsource and a camera that captures an image of a measurement subjectplaced in an optical path that lies between said camera itself and saidlight source, comprising: subtracting a plurality of frame imagescaptured by said camera during an OFF period of said light source from aplurality of frame images captured by said camera during an ON period ofsaid light source, the number of frame images captured by said cameraduring the OFF period being the same as the number of frame imagescaptured by said camera during the ON period, and integrating thedifferences between their images.